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In Fusion mode, enemies are even more powerful, and you can play as Fusion Suit Samus.

—In-game description

Samus Returns Samus Data Screen and Fusion Mode

Fusion Mode highlighted on the Samus Data Screen.

Fusion Mode is a difficulty level that is unlockable in Metroid: Samus Returns. Featuring Samus Aran's Fusion Suit from Metroid Fusion, Fusion Mode also increases the game's difficulty to a greater degree than Hard Mode, with enemies dealing four times the damage they would on Normal Mode, which is a challenging level itself. It is unlocked by completing the game once, and then tapping the Metroid amiibo on the Nintendo 3DS. Initially, this incited controversy from fans, who criticized Nintendo for locking a harder difficulty behind rare amiibo. In response, Nintendo clarified that a separate Hard Mode is included in the game, unlockable after completing it once on normal difficulty, and does not require amiibo.[1] [2]

At Gamescom 2017, the Metroid: Samus Returns Overview Trailer showcased everything that Metroid amiibo figures unlock in the game, including Fusion Mode. Yoshio Sakamoto told Eurogamer that Fusion Mode is "significantly" more difficult than Hard Mode, but did not go into detail as he wanted fans to experience it for themselves. He did reveal however that in Fusion Mode, Save Stations are still present.[3]